The present invention relates to a vehicle electrical system comprising a generator operative to deliver electrical power to an electrical load, a voltage regulator which maintains the generator output voltage at a regulation voltage, and a control device which utilizes a switch module in place of the conventional diode trio, further complementing the voltage regulator's operation, i.e., field switching, and improving transient recovery due to sudden variations in the electrical load. Utilizing the switch module instead of the conventional diode trio, the control device is capable of swiftly suppressing voltage transients. In an over voltage condition, the control device utilizes the switch module to clip the generator output voltage to within an acceptable level by switching on the switch module, thereby, dissipating stator current within the stator windings. In an under voltage condition, the control device utilizes the switch module to temporarily boost the generator output power by switching on/off the switch module at a frequency which is higher than the generator RPM. The control device may also utilize the switch module during normal operation to increase system efficiency by switching on/off the switch module synchronously, thereby, reducing electrical power loss within the switch module. The control device and its method of operation make it possible to use low-power transistors in the switch module. There are several advantages to this architecture including but not limited to implementation of the control device without the need for changing the generator components, reduction in the overall dimension of the generator or voltage regulator, increased system efficiency, and realization of improved transient recovery utilizing inexpensive components.
A typical vehicle electrical system comprises a generator and a voltage regulator. The generator operates to provide electrical power to the electrical load at a regulation voltage according to the voltage regulator parameters. The voltage regulator operates to maintain the system voltage at the regulation voltage by switching on/off the generator field coil in accordance with the electrical load requirements and generator RPM. In a conventional system, utilizing a generator with three stator windings, the field coil, as is known to artisans of ordinary skill, is coupled to the generator stator windings via three diodes, commonly referred to as the diode trio. Conventional generators use the diode trio for two main reasons. The first reason is to make use of the residual magnetism of the generator in order to “wake up” the regulator when the generator starts rotating. The concept is known in the industry as “self-energizing regulator” and refers to the fact that there is no need for a dedicated energize signal, which otherwise usually comes through an additional wire and is activated by the vehicle ignition switch. The second reason for using the diode trio is to reduce current flow through the positive rectifier diodes in the main rectifier assembly, since the field current goes directly from stator windings to the field coil through the diode trio terminal.
This invention improves the conventional system by substituting the diode trio with a switch module comprising bidirectional solid state switches (MOSFTs or IGBTs with anti-parallel body diodes) and making use of the bidirectional capability of the switches through a control algorithm, which adds an important advantage, namely, faster and better transient response, resulting in very tight voltage regulation even under extreme load transients. As an additional benefit, the MOSFETs/IGBTs can be used to improve the efficiency of field circuit through synchronous rectification.
For heavy duty generators operating at high speed and load, electrical load variations can lead to unacceptably large voltage transient levels and duration. For instance, over voltage condition may be large and long enough to be interpreted by the voltage regulator, equipped with an over voltage protection circuit, as a genuine over voltage condition, causing it to deactivate itself. Alternatively, the over voltage condition may be tolerated by the voltage regulator but nevertheless be long enough to the detriment of certain electrical components within the electrical system. Under voltage conditions, due to sudden application of electrical loads, can also lead to system malfunction. For instance, certain electrical components in the vehicle electrical system may reset, reinitialize, or timeout due to low system voltage.
As the number of electrical components in vehicles increases, electrical power consumption increases accordingly. As a result, the vehicle electrical system must use high power generators that can produce sufficient electrical power to meet the demand. High power generators have correspondingly higher energy transients than lower power generators. Transient voltages associated with electrical load variations in such electrical systems can be detrimental to electrical components in the electrical system.
A typical vehicle electrical system includes electrical components that comprise semiconductor devices, such as power field effect transistors (FETs), smart power integrated circuits (ICs), microcontroller units (MCUs), digital signal processors (DSPs), memory, analog ICs, and numerous discrete devices. Sudden load variations in the electrical system due to sudden connection/disconnection of the electrical loads can destroy or otherwise cause malfunction in such devices. These transients are the most potentially destructive transients in the vehicle electrical system due to the combination of high voltage and high energy.
Under voltage condition occurs when one or more electrical loads are suddenly switched on and the generator is unable to produce electrical power fast enough to supply the electrical loads. Over voltage condition occurs when one or more electrical loads are suddenly disconnected and the generator is unable to dissipate the electrical energy in the generator field and stator coils fast enough to keep the output voltage within an acceptable voltage range. Although vehicle electrical systems ordinarily include one or more electrical energy sources such as batteries which, to a certain degree, improve the under voltage and over voltage conditions, extreme voltage transients still affect the power quality in such electrical systems. Furthermore, there are some applications where the vehicle electrical system does not include batteries which exacerbate the transient voltage variations due to sudden connection/disconnection of the electrical loads.
Batteries in a vehicle electrical system operate to provide electrical power to the electrical loads when the vehicle engine is turned off and/or when the vehicle engine is turned on but the generator is incapable of generating sufficient electrical power at the operating speed (RPM) to meet the demand, such as is the case when a high electrical power consuming device like an air conditioning unit is switched on. Batteries also act as reservoirs where excess electrical power can be stored, such as is the case when the air conditioning unit is suddenly switched off. Such sudden demand and supply of electrical power in the electrical system can occur even when the vehicle engine is operating at the rated RPM.
For instance, a vehicle electrical system including a generator that is rated to generate 500 Amps at 5000 RPM will, momentarily, experience a dip in the system voltage when a large electrical load is suddenly switched on due to the slow response time of the generator which may last in the order of hundreds of milliseconds. Similarly, a sudden disconnection of the electrical load at the above mentioned RPM, will give rise to a spike in the system voltage that may last for the same time period. These under voltage and over voltage conditions occur even in the presence of one or more batteries in the electrical system.
Not only are these transient conditions detrimental to the electrical components in the vehicle electrical system, repetitive battery under charge and over charge is detrimental to the batteries. Furthermore, the effects of the under voltage and over voltage conditions on the electrical components are intensified in batteryless applications. Consequently, there is a need for a control device that operates to improve the power quality of the electrical system by suppressing voltage transients due to sudden variations in the electrical loads.
Although various systems have been proposed which touch upon some aspects of the above problems, they do not provide solutions to the existing limitations in providing high quality electrical power within a vehicle electrical system. For example, in Muller et al., U.S. Pat. No. 6,924,629, a device and a method for the control of a generator are disclosed in which the rectifier bridge connected to the generator can be temporarily short-circuited, as a result of which power is temporarily stored in the stator inductors, which results in higher phase voltages. Suitable selection of the control frequency for a transistor, which makes the short-circuiting of the diode bridge possible, allows an output voltage of the generator to be set to the desired voltage level which is clearly higher than the conventional vehicle electrical system voltage. The diode bridge itself can be replaced by controllable switching elements (transistors) and a voltage adjustment is implemented using suitable controls. Muller is limited in that it uses the generator's main rectifiers to control the stator current which must necessarily be made up of high-power electrical components.
Koelle et al., U.S. Pat. No. 6,353,307, discloses a controlled rectifier bridge for a generator having a plurality of phase windings and one exciter winding is constructed as a self-controlled rectifier bridge with MOS field effect transistor. To allow the use of such a rectifier bridge upon a fast load reduction with an attendant load-dump voltage, a voltage protection circuit is employed that feeds the energy, stored in the exciter winding upon a fast shutoff, back into the battery, thus deexciting the exciter winding. Upon a fast load reduction, the generator windings are short-circuited by suitable triggering of the low- or high-side transistors. Similar to Muller, the rectifiers require high-power electrical components and furthermore, it does nothing to improve undervoltage conditions.
In today's modern vehicles, the vehicle electrical system comprises a large number of electrical components that consume large amounts of electrical power. Consequently, vehicle electrical systems use high power generators to meet the high electrical power requirement. Additionally, the vehicle electrical system incorporates electrical devices that are often sensitive to voltage fluctuations in the electrical system. As a result, the vehicle electrical system must provide high electrical power while minimizing the transient effects due to connection and/or disconnection of the electrical components. This requires the vehicle electrical system to be capable of rapidly suppressing any under voltage and over voltage conditions that may occur as a result of such connection/disconnection, before the electrical components malfunction or become inoperative. As a simple, yet efficient, alternative to existing technologies, the present invention offers a vehicle electrical system capable of providing high electrical power of high quality to multiple electrical components within the vehicle electrical system.
In particular, the control device of the present invention utilizes a switch module instead of the traditional diode trio which couples the generator stator windings to the field coil. The control device implements a control algorithm which monitors the field current, stator current, and output voltage and switches the switch module to controllably short the stator windings so as to suppress voltage transients. By monitoring the field current and output voltage and timely switching the switch module, the control device may utilize low-power transistors to achieve the same result where traditionally use of high-power components was required. The present architecture also lends itself to be used during normal operation by further monitoring the stator current and synchronously switching the switch module to improve system efficiency.